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nsfs: mark dentry with DCACHE_RCUACCESS

Andrey reported a use-after-free in __ns_get_path():

  spin_lock include/linux/spinlock.h:299 [inline]
  lockref_get_not_dead+0x19/0x80 lib/lockref.c:179
  __ns_get_path+0x197/0x860 fs/nsfs.c:66
  open_related_ns+0xda/0x200 fs/nsfs.c:143
  sock_ioctl+0x39d/0x440 net/socket.c:1001
  vfs_ioctl fs/ioctl.c:45 [inline]
  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:685
  SYSC_ioctl fs/ioctl.c:700 [inline]
  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691

We are under rcu read lock protection at that point:

        d = atomic_long_read(&ns->stashed);
        if (!d)
                goto slow;
        dentry = (struct dentry *)d;
        if (!lockref_get_not_dead(&dentry->d_lockref))
                goto slow;

but don't use a proper RCU API on the free path, therefore a parallel
__d_free() could free it at the same time.  We need to mark the stashed
dentry with DCACHE_RCUACCESS so that __d_free() will be called after all
readers leave RCU.

Fixes: e149ed2 ("take the targets of /proc/*/ns/* symlinks to separate fs")
Cc: Alexander Viro <>
Cc: Andrew Morton <>
Reported-by: Andrey Konovalov <>
Signed-off-by: Cong Wang <>
Signed-off-by: Linus Torvalds <>
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congwang authored and torvalds committed Apr 19, 2017
1 parent 80d136e commit 073c516ff73557a8f7315066856c04b50383ac34
Showing with 1 addition and 0 deletions.
  1. +1 −0 fs/nsfs.c
@@ -91,6 +91,7 @@ static void *__ns_get_path(struct path *path, struct ns_common *ns)
return ERR_PTR(-ENOMEM);
d_instantiate(dentry, inode);
dentry->d_flags |= DCACHE_RCUACCESS;
dentry->d_fsdata = (void *)ns->ops;
d = atomic_long_cmpxchg(&ns->stashed, 0, (unsigned long)dentry);
if (d) {

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